Figure out what to do about optional dictionary members that are also dictionaries

[Manishearth]

Our last webidl update explicitly excludes bug 1368949 (see also: bug 1493860 , bug 1493798) This basically means that the following doesn't work:

dictionary Foo {required bool thing; };
dictionary Bar {Foo foo;}

since we treat null=empty for dictionary members, which doesn't quite work for dictionaries that can't be empty

This is also related to heycam/webidl#76

It's a bit of a mess, I tried to fix it and ended up realizing that I'm not quite sure what the fix should be (just applying the patch from bug 1368949 isn't enough, since our media and domquad code breaks. The Gecko patches don't seem to distinguish between dictionaries with required members and those without, so codegen overcorrects.

Ideally, we continue to handle the following case the same way:

dictionary Foo {bool thing = false; };
dictionary Bar {Foo foo;}

with an unspecified foo defaulting to {thing: false}, but if the member is required, then we instead create a nullable field.

A simple fix is to apply the patch and just change our DOM code to accept the spuriously generated Options in a couple places.